1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an active matrix type liquid crystal display device.
2. Description of Prior Art
FIG. 5 is a schematic longitudinal cross-sectional view illustrating the main portion of a conventional active matrix type liquid crystal display device 50.
In the liquid crystal display device 50, a liquid crystal display cell 3 is arranged between two polarizers 1 and 2. It is arranged so that the axes of polarization of the two polarizers are parallel to each other. The liquid crystal display cell 3 includes a transparent insulating substrate 6 of glass on which Thin Film Transistors (TFTs) 4 and transparent conductive picture element electrodes 5 for displaying picture elements are formed. It further includes another transparent insulating substrate 8 of glass on which transparent conductive electrodes 7 are formed so as to oppose to the electrodes 5 of the former substrate 6. Finally, a liquid crystal layer 9 is arranged between the transparent substrates 6 and 8. The TFTs 4 are formed in a matrix form so as to correspond to the respective picture element electrodes 5.
An orientation film 10 is formed on the inner surface of the transparent substrate 6 so as to cover the TFTs 4 and the picture element electrodes 5. Further, an orientation film 11 is formed on the inner surface of the transparent substrate 8 so as to cover the electrodes 7. Nematic liquid crystal molecules contained in the liquid crystal layer 9 are twisted by 90.degree. between the orientation films 10 and 11, while zero voltage is applied between the electrodes 5 and 7. Namely, the nematic liquid crystal molecules in the layer 9 are set in the twisted nematic mode. Shading films 12 are formed between each of the electrodes 7 and the substrate 8 so as to face the respective TFTs 4. Further, color filter layers 13 are also formed on the inner surfaces of the substrate 8 so as to face the respective portions of the picture element electrodes 5 where the TFTs 4 are not formed. In FIG. 5, arrows A and B represent directions of the polarization axes of the polarizer 1 and the polarizer 2, respectively.
FIG. 6 illustrates an enlarged schematic longitudinal cross-sectional view showing the TFT 4 used as a switching element for driving the liquid crystal layer 9 in the liquid crystal display device 50.
In FIG. 6, a gate electrode 14 of a metal film is formed on the surface of the transparent substrate 6, and a gate insulating film 15 is formed thereon. Furthermore, an amorphous silicon hydride film 16 functioning as an active layer is formed on the film 15. Also n+ amorphous silicon hydride films 17 and 18 are formed thereon. In FIG. 6, a numeral 19 denotes an insulating film, and a numeral 20 denotes a metal film.
In the aforementioned liquid crystal display device 50 constructed as described above, a light is emitted from a light source (not shown) arranged on the side of the outer surface of the polarizer 1 as indicated by arrows P. Further, the emitted light is converted to a linearly polarized light by the polarizer 1. The linearly polarized light is transmitted through the transparent substrate 6, the electrode 5, the liquid crystal layer 9, the electrode 7, the color filter layer 13 and the transparent substrate 8, sequentially, and is incident to the polarizer 2.
As described above, since the liquid crystal layer 9 is set in the twisted nematic mode and the respective polarization axes of the polarizers 1 and 2 are in parallel to each other, when zero voltage is applied between the electrodes 5 and 7, the polarization axis of the light transmitted through the polarizer 1 from the light source is twisted by 90.degree. according to the rotary polarization characteristic of the liquid crystal layer 9. Therefore, the polarization axis thereof becomes orthogonal to that of the polarizer 2. This results in the light being prevented from being transmitted through the polarizer 2. On the other hand, since the rotatory polarization characteristic of the liquid crystal layer 9 is dissolved when a predetermined voltage is applied between the electrodes 5 and 7, the polarization axis of the light incident to the polarizer 2 is coincident with that of the polarizer 2. This results in the light being transmitted through the polarizer 2.
The voltage to be applied between the electrodes 5 and 7 is controlled by the TFTs 4. The picture elements corresponding to the electrodes 5 and 7 between which the predetermined voltage is applied according to the above operation become a transparent state. Further, a colored display is effected by the transmitted light colored by the color filter layer 13.
By the above operation, the light transmitted through the polarizer 1 and the transparent substrate 6 is incident to the TFTs 4, including the amorphous silicon hydride film 16 having a high photoconductivity for a visible light. In this case, since the gate electrode 14 of metal film functions as a shading film, increase of the OFF current in the TFTs 4, caused by the photoconductivity of the film 16, is prevented. Particularly, in the case that images, for example, are projected onto a screen through a projecting lens in a magnification mode by use of the active matrix type liquid crystal display device using the TFTs 4 having the aforementioned amorphous silicon hydride film 15 as switching elements, an extremely strong light is required as the light source in order to produce a bright display. In this case, the illuminance of the light incident to the liquid crystal display device 50 is in the range of several hundred thousand to several million 1x. In this case, since the gate electrode 14 of the TFT 4 also functions as the shading film, increase of the OFF current in the TFT 4 caused by the photoconductivity is prevented. This results in the degradation of the display characteristics being avoided.
As described above, in the aforementioned conventional active matrix type liquid crystal display device, a display mode is used wherein the light is not transmitted at the time of the application of zero voltage. Further, the light is transmitted by dissolving the rotatory polarization characteristic of the liquid crystal layer 9 when a predetermined voltage is applied. In the display device, because the optical rotatory dispersion is caused in the liquid crystal layer 9, a problem exists in that different electric optical characteristics are obtained, respectively, for lights having different wavelengths such as red, green and blue.
FIG. 7 illustrates the dependency of the transmittance on the root mean square value of the applied voltage with a parameter of the wavelength of light (red, green and blue) in the aforementioned active matrix type liquid crystal display device 50. In FIG. 7, the vertical axis represents the root light transmittance and the horizontal axis represents the root means square value of the applied voltage.
As is apparent from FIG. 7, in the display mode, at the time of the application of approximately zero voltage, when the applied voltage is close to the voltage of black, for example, the liquid crystal layer 9 is in the twisted nematic mode, the leak light caused by the optical rotatory dispersion increases. This results in the color reproductivity being lowered.
In the aforementioned active matrix type liquid crystal display device, if the projecting display is performed for a relatively long time or the display is performed with use of a back lighting apparatus such as a fluorescent lamp, the threshold voltage of the TFT 4 is shifted with lapse of the light projecting time. This results in the degradation of the display characteristic.
FIG. 3 illustrates the relationship between the light projecting time (hours) and the shift amount .DELTA.V.sub.TH (volts) of the threshold voltage of the TFT 4, wherein the visible light having an illuminance of 700,000 1x is projected onto the TFT 4. In FIG. 3, the curve l1 illustrates the shift amount .DELTA.V.sub.TH of the threshold voltage of the TFT 4 in the conventional active matrix type liquid crystal display device 50. As is apparent from the curve l1 of FIG. 3, with lapse of the light projecting time, the threshold voltage of the TFT 4 is gradually shifted.
In order to solve the problem of the degradation of the color reproductivity caused by the optical rotatory dispersion, a method for equalizing the color reproductivities for the respective lights having the wavelengths of red, green and blue has been proposed. According to the method, the color reproductivities for red, green and blue colors are equalized by setting respective gaps (d) of the portions of the liquid crystal cell corresponding to individual picture elements for displaying the red, green and blue colors so as to make the ratio .DELTA.n.multidot.d/.lambda. constant n represents the birefringence of the liquid crystal and .lambda. represents the wavelength of light. (See, for example, Hotta et al.: SID' 86 Digest. p. 296 (1986)).
However, in this method, it is difficult to set the gap (d) of the liquid crystal cell exactly for every color of red, green and blue. This results in the cost for the mass production being extremely increased.
On the other hand, the present inventors have discovered that the shift of the threshold voltage of the TFT 4 due to the light projection for a relatively long time is caused by a portion 16a of the amorphous silicon hydride film 16 uncovered by the gate electrode 14 (See FIG. 6). However, according to the usual manufacturing process for the TFT, it is extremely difficult to completely remove aforementioned projected portion 16a without providing a bad influence to the characteristics of the TFT.